Space occupancy strategy regulates closed pores to enhance the plateau capacity of hard carbon

Abstract

Structural regulation of closed pores represents a critical pathway for enhancing the plateau capacity of hard carbon (HC), a key determinant for achieving high energy density in sodium-ion batteries (SIBs). However, designing closed pores suitable for SIBs remains a significant challenge. Herein, we present a space occupancy strategy utilizing partially rigid-branched aldehydes as sacrificial spacers during precursor synthesis to engineer a closed-pore-rich architecture in phenolic resin-derived HC. Rigid branches exert steric hindrance and generate decomposition-induced voids at low carbonization temperatures, suppressing graphitization while creating nanovoids for closed-pore formation. The optimized HC delivers an exceptional reversible capacity of 398.8 mAh g⁻¹ at 20 mA g⁻¹, with an ultrahigh plateau capacity of 303.3 mAh g⁻¹. The full cell achieves 284.40 Wh kg⁻¹ energy density with 89.9% capacity retention over 120 cycles. Multiple in situ characterizations and structure–property correlations confirm that closed-pore structures govern sodium storage in the low-voltage plateau region. This work establishes a viable design paradigm for high-plateau-capacity HC anodes.